<target>      <source>         <keyfile>      <options>
part1_crypt   /dev/disk/...    crypt_disks    luks,keyscript=decrypt_keyctl
part2_crypt   /dev/disk/...    crypt_disks    luks,keyscript=decrypt_keyctl

dd if=/dev/urandom of=<path to key file> bs=1024 count=1
chmod u=rw,g=,o= <path to key file>

cryptsetup luksAddKey <path to encrypted device> <path to key file>

<target>      <source>         <keyfile>                  <options>
root_crypt    /dev/disk/...    none                       luks
part1_crypt   /dev/disk/...    <path to key file>         luks